Communication of data over the Public Switched Telephone Network (PSTN) is becoming an increasingly important aspect of computing. In particular, communication between portable computers and office computers or networks has increased substantially along with advances in portable computers. Such communications allow a user remote access to data stored on a host computer system. Advances in cellular communications have increased the ability of people to transact business in locations away from a hardwire connection to the telephone company's central office. Consequently, the need to exchange data through a cellular or other radio telephone transceiver has increased.
In one form of the prior art, a standard touch-tone or rotary dial telephone set was interfaced to a radio transceiver by means of an interface which provides a simulation of a telephone central-office. Such systems, which will be referred to as Telco emulators, are disclosed in U.S. Pat No. 4,737,975 to Shafer and U.S. Pat. Nos. 4,658,096 and 4,775,997 to West and Shafer. These patents disclose essential features required of an interfacing means between an ordinary touch-tone or rotary dial telephone set and a cellular telephone or other radiotelephone transceiver in order to provide for the communication of voice between the telephone set and the radiotelephone transceiver and to provide for establishing a call through the radiotelephone transceiver by the telephone set. Telco emulators have been used to connect computers to radio transceivers via modems. For example, U.S. Pat. No. 4,837,800 to Freeburg, et al. shows a Telco emulator, the Motorola "CELLULAR CONNECTION" used to connect a modem to a cellular radio transceiver. While such an interface may be used to interface a computer to the radio transceiver via a conventional telephone-line modem, the features described are more complex than required by a modern microprocessor-based modem. The extra complexity adds to the size and weight of such interfacing means, making it less desirable in a portable environment.
While this prior art provides a simple single-pair interface between the telephone set and the interface device, the requirement of the standard telephone set for DC current, dial tone, audible tone ring-signal, and conversion of touch-tone or rotary dialing information as well as the requirement for the interface means to automatically determine the last digit dialed in order to signal the radiotelephone device to initiate the connection adds complexity, weight, and cost to the interface. This is a major disadvantage for portable computers for a number of reasons. First, computer manufacturers are reluctant to install special, extra-cost, modems in portable computers, knowing that only a small percentage of the users of the portable computers will desire to communicate over cellular or other radiotelephone facilities. Second, minimizing size and weight is very important to portable computer users.
In other prior art disclosed in U.S. Pat. No. 5,127,041 to O'Sullivan, means for interfacing a modem device to a cellular radio transceiver is shown which requires the use of separate digital-control and analog busses connecting the modem to the interface device. This approach, while demonstrably workable and providing connection between the modem device and the cellular telephone using the modem's PSTN connector, requires additional wires in the connection. This poses regulatory and safety issues (see Part 47 CFR Subpart 68) and increases the complexity of the connection between the modem and the interface device. This approach also requires special circuits in the modem to provide the digital control signals, increasing the size, complexity, and cost of the modem.
U.S. Pat. No. 5,127,041 also calls for a "Generic Control Signals" from the modem device to be translated by the interface device to the particular digital signaling sequences required for control of a particular radiotelephone transceiver. While this feature provides for connection of multiple different radiotelephone transceivers to a single modem via different interface devices for each type of radiotelephone transceiver, it is not required by a modem micro-processor based modem which can generate different control signals and sequences as required under the control of programs executed by the modem and stored in the modem or the computer to which the modem is attached. Eliminating the requirement for "Generic Control Signal" translation from the interface provides a wider range of possible attachments and a lower-cost interface device.
Radiotelephone interfacing systems not related to modems also exist in prior art. U.S. Pat. No. 4,852,146 to Hathcock et al. discloses an auxiliary dialing system between a handset and a cellular radio transceiver, but this system also uses more than a single pair of wires and requires multiple signaling circuits.
Therefore a need has arisen for a simplified interface for establishing communication between a modem and a radiotelephone transceiver which does not add significant cost or weight to the modem.